<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }
        li {
            list-style: none;
        }
        .header {
            background-color: darkgrey;
        }

        .footer {
            background-color: peru;
            height: 100px;
        }
        .cell {
            float: left;
            height: 50px;
            width: 50px;
            background-color: gold;
            border: 1px solid red;
        }


        .clearfix:before, .clearfix:after {
            content: "";
            display: table;
        }
        .clearfix:after {
            clear:both;
        }
        .clearfix {
            *zoom: 1 ;
        }

    </style>
</head>
<body>
<div class="header clearfix">
    <div class="cell">Cell1</div>
    <div class="cell">Cell2</div>
    <div class="cell">Cell3</div>
</div>
<div class="footer">Footer</div>
</body>
</html>